Changes In Surface State Research Articles

Effects of High-Humidity Aging on Platinum, Palladium, and Gold Loaded Tin Oxide—Volatile Organic Compound Sensors

This study is an investigation of high-humidity aging effects on the total volatile organic compound (T–VOC) gas-sensing properties of platinum, palladium, and gold-loaded tin oxide (Pt,Pd,Au/SnO2) thick films. The sensor responses of the high-humidity aged Pt,Pd,Au/SnO2, a non-aged Pt,Pd,Au/SnO2, and a high-humidity aged Pt/SnO2 to T–VOC test gas have been measured. The high-humidity aging is an effective treatment for resistance to humidity change for the Pt,Pd,Au/SnO2 but not effective for the Pt/SnO2. The mechanism of the high-humidity aging effects is discussed based on the change of surface state of the SnO2 particles.

Comparative study of plasma and ion-beam treatment to reduce the oxygen vacancies in TiO2 and recombination reactions in dye-sensitized solar cells

Oxygen plasma and ion-beam treatments of nano-structured TiO2 were carried out to enhance the efficiency of dye-sensitized solar cells (DSSCs). Relative changes in surface states and recombination reactions were examined. Comparative studies showed that the oxygen ion-beam treatment was more effective in reducing the number of oxygen vacancies in TiO2 and the recombination reactions between TiO2 conduction band electrons and I3- ions in the electrolyte than the plasma treatment. Oxygen ion-beam treated TiO2 film in the DSSC showed a 23mV increase in the open-circuit voltage (Voc) and an improvement in the photo-conversion efficiency from 5.05% to 6.64%.

Using hybrid silica-conjugated TiO2 nanostructures to enhance the efficiency of dye-sensitized solar cells

Abstract In this study, hybrid silica-conjugated TiO2 photoelectrodes were developed in order to enhance the efficiency of a dye-sensitized solar cell. The relative changes in surface crystallite size and chemical surface states of TiO2 composites were investigated by XRD, XPS, and UV–vis spectroscopy. Therein, the chemical compositions of the nanostructured photoelectrode surfaces were observed to significantly change when the glass powder Si atoms became chemically bonded with the Ti atoms on the photoelectrode surface without appreciable changes to the crystalline structure of TiO2. Furthermore, a significant conversion of Si–Ox into Si–O at the surface of the photoelectrode was observed following the addition of glass powder, which confirms the covalent bonding of Si and Ti atoms into Ti–O–Si. A maximum cell efficiency (η from 5.8% to 8.5%) was observed when 2 wt% of the low-temperature glass powder was added to the TiO2 with a constant amount of dye loading. This observed peak in solar cell efficiently is most likely due to an increase in light harvesting, which is a result of an enhancement of light scattering and the coordination between Ti and Si to establish a Ti–O–Si bond.

Changes in the valence band structure of as‐grown InN(0001)‐2 × 2 surfaces upon exposure to oxygen and water

AbstractWe investigated the surface chemistry and valence band (VB) structure of as‐grown thin InN(0001)‐2 × 2 films as well as their change upon the exposure to oxygen and water. The InN films were grown by plasma‐assisted molecular beam epitaxy (PAMBE) and in situ characterized by reflection high electron energy diffraction (RHEED) and photoelectron spectroscopy (UPS, XPS). The oxygen and water exposure was directly performed on the as‐grown, contamination‐free InN surfaces at room temperature and leads to changes in the chemical surface states as well as the electronic properties. For 2 × 2 reconstructed InN surfaces one observes directly after growth a surface state at the Fermi‐edge which decreases continuously with oxygen and water exposure. Furthermore, two oxygen related electronic states develop in the VB at binding energies at around 5 and 10 eV. For water exposure a third weak state around 8 eV is additionally observed. The impact of oxygen and water on the work function Φ as well as the variation of surface band bending was investigated. In both cases for initially 2 × 2 reconstructed surfaces a reduction in the downward band bending is found, while Φ increases in the case of oxygen exposure but in the case of interaction with water a reduced work function is observed. The oxygen uptake rates reveal a higher reactivity of water with InN surfaces compared to oxygen. Furthermore, during oxidation and water exposure different chemical oxygen bonds are formed, but a direct assignment to In–O or N–O bonds is difficult due to changes in the In3d and N1s XPS core level peak shape.

A comparative DFT study of electronic properties of 2H-, 4H- and 6H-SiC(0001) and SiC() clean surfaces: significance of the surface Stark effect

The electric field, uniform within a slab, emerging due to Fermi level pinning at both sides of the slab, is analyzed using DFT simulations of SiC surface slabs of different thicknesses. It is shown that for thicker slabs the field is nonuniform and this fact is related to the surface state charge. Using the electron density and potential profiles, it is proved that for high-precision simulations it is necessary to take into account a sufficient number of SiC layers. We show that the use of 12 diatomic layers leads to satisfactory results. It is also demonstrated that the change of the opposite side slab termination, both by different types of atoms or by their location, can be used to adjust the electric field within the slab, creating a tool for simulation of surface properties, depending on the doping in the bulk of the semiconductor. Using these simulations, it was found that, depending on the electric field, the energy of the surface states changes in a different way than the energy of the bulk states. This criterion can be used to distinguish Shockley and Tamm surface states. The electronic properties, i.e. energy and type of surface states of the three clean surfaces: 2H-, 4H-, 6H-SiC(0001) and SiC(), are analyzed and compared using field-dependent DFT simulations.

Hysteresis in the quantized current characteristics of single-electron-transport device

We discovered a hysteresis feature in the quantized current characteristics of single-electron-transport device driven by surface acoustic wave. At gate voltages beyond the conductance pinch-off, the curves of the quantized current versus the gate voltage increasing do not coincide with that of the decreasing. At the fixed gate voltage, the current for the gate voltage decreasing is larger than that of the gate voltage increasing. The change of surface state distribution in the etching sidewalls is considered as the physical origin of the hysteresis characteristics. In addition, we estimated the density of surface states changes about 1.1 × 10 13 m − 2 at the same gate voltage with twice sweeping in opposite direction.

Change in Surface States of Amorphous Carbon Nitride Films after Exposure to Oxygen Plasma

Amorphous carbon nitride (a-CNx) thin films were deposited by rf-reactive sputtering method using a graphite target, and after deposition the films were exposed to oxygen plasma. The effect of the oxygen plasma exposure on the morphology and chemical bonding states of the film surface has been studied. Film composition and the chemical bonding states were analyzed by X-ray photoelectron spectroscopy (XPS). Film surface was observed by atomic force microscopy (AFM). AFM observations have revealed that the as-deposited film surface is uniformly covered with particle-like features in the early stage of deposition and the surface changes to be covered with broccoli-like features with increasing the deposition time and correspondingly the surface roughness increases, while after exposure to oxygen plasma, the film surface was etched selectively and the surface roughness increases with the plasma exposure time. It should be noted that the etching behavior depends on the film deposition temperatures. XPS studies have shown that after exposure to oxygen plasma the change in the bonding states in the films prepared at 853 K is different from that in the films prepared at RT.

Change in surface states of Ag(111) thin films upon adsorption of a monolayer of PTCDA organic molecules

The change in the electronic structure of silver thin films of different thicknesses with the Ag( 111) orientation due to the interaction with an adsorbed monolayer of ordered organic molecules of 3,4,9,10-perylene-tetracarboxylic acid dianhydride (PTCDA) has been investigated in terms of density functional theory. It has been shown that one of the two surface states of the pure films transforms into an unocc upied interface state due to the interaction so that all the main features of the initial state are retained. The relation of the resulting state to the unoccupied state experimentally observed in the PTCDA/Ag( 111 ) system by scanning tunneling and two-photon photoemi ssion spectroscopy has been discussed.

Optical tuning of molecule‐induced magnetism at non‐magnetic semiconductor nanoparticles

AbstractWe have demonstrated that it is possible to reversibly photo‐control the room temperature magnetism in a diluted magnetic semiconductor based on zinc oxide nanoparticles coated with azobenzene‐derivatized thiols (Azo‐ZnO NPs). These photochromic ligands provide a photo‐function and at the same time act as a trigger for the generation of ferromagnetism at ZnO–S interface. UV irradiation increases the saturated magnetization of the Azo‐ZnO NPs due to trans‐ to cis‐isomerization in azobenzene‐derivatized ligands, and following irradiation with visible light the magnetization is decreased due to inverse isomerization. X‐ray photoelectron spectroscopy reveal that the photo‐induced magnetization changes are strongly related to photo‐induced changes in the chemical state of zinc atom in the ZnO. These photo‐magnetic effects can be attributed to photo‐induced changes in surface states of the ZnO due to photoisomerization in azobenzene ligands, which is accompanied by alterations in the lowest unoccupied molecular orbital level of the ligand, since magnetic properties of ZnO depend on carrier concentration, which correlated with surface states of the ZnO.magnified image

Ab initio studies of electronic properties of bare GaN(0001) surface

Laplace correction was used for solutions of Poisson equation in density functional theory (DFT) slab simulations of GaN (0001) surface. In accordance to the scenario of Meyer and Marx [Phys. Rev. B 67, 35403 (2003)], the electric field, emerging within the slab, is determined by the condition that Fermi energy is equal to the energies of valence band maximum and conduction band minimum at both sides of the slab. In contrast to their predictions that the field is identical for different slab thicknesses, it was found from DFT calculations that the electric potential difference between two sides of the slab is kept constant. Therefore, the energy of the slab does not diverge for large slab thickness. It was also found that, in most cases, the field in the slab is determined by pinning by surface states. Thus the change in opposite side slab termination, both by different types of atoms and by their location, can be used to change electric field in the slab, creating a tool that can be used to simulate the change in surface properties due to doping of the bulk. It was shown that, depending on the electric field, the energy of surface states changes in the way different from the band states. This change could be large, comparable to the bandgap. Despite the fact that the Fermi energy can be pinned to surface states, it may change its energy with respect to valence band and conduction band as a function of the doping of bulk semiconductor. These results are in agreement with the experimental data showing variation in Fermi energy at the GaN (0001) surface in function of the doping in the bulk. It was also shown using the electron density and potential profiles that ten Ga–N atomic layers are necessary to achieve small quantum overlap of the surface states of both sides of the slab, which is required for high precision simulations of the surface.

Combined use of optical and radar satellite data for the detection of tillage and irrigation operations: Case study in Central Morocco

The objective of this study is to present a new application of optical and radar remote sensing with high spatial (∼10 m) and temporal (a few days) resolutions for the detection of tillage and irrigation operations. The analysis was performed for irrigated wheat crops in the semi-arid Tensift/Marrakech plain (Central Morocco) using three FORMOSAT-2 images and two ASAR images acquired within one week at the beginning of the 2005/2006 agricultural season. The approach we developed uses simple mapping algorithms (band thresholding and decision tree) for the characterisation of soil surface states. The first images acquired by FORMOSAT and ASAR were processed to classify fields into three main categories: ploughed (in depth), prepared to be sown (harrowed), and not ploughed-not harrowed. This information was combined with a change detection analysis based on multitemporal images to identify harrowing and irrigation operations which occurred between two satellite observations. The performance of the algorithm was evaluated using data related to land use and agricultural practices collected on 124 fields. The analysis shows that drastic changes of surface states caused by ploughing or irrigation are detected without ambiguity (consistency index of 96%). This study provided evidence that optical and radar data contain complementary information for the detection of agricultural operations at the beginning of agricultural season. This information could be useful in regional decision support systems to refine crop calendars and to improve prediction of crop water needs over large areas.

Combined insitu EXAFS and electrochemical investigation of the oxygen reduction reaction on unmodified and Se-modified Ru/C

In this work we study the kinetics of the oxygen reduction reaction on carbon-supported Ru nanoparticles modified with various amounts of Se. Rotating disk electrode is used to determine kinetic currents for the ORR in 0.1M H2SO4 at 298K and O2 partial pressures from 1 to 0.01atm. The dependence of the ORR activity on Se/Ru ratio shows volcano-type behavior with ca. 10 fold increase of the mass activity at 0.1<Se/Ru<0.3. The reaction order in O2 is close to 1 in the interval of overpotentials from 0.4 to 0.7V, and is independent of the presence of Se. Regardless the amount of Se, the Tafel slope demonstrates continuous increase from ca. 70mV/dec at 0.4V to ca. 140mV/dec at 0.6V overpotential. In situ EXAFS spectra are measured at Ru K-edge (in the transmission mode) and Se K-edge (in the fluorescence mode) in argon and oxygen saturated 0.1M H2SO4 solutions in the interval of electrode potentials from 0.050 to 0.750V RHE. The data are used to explore the surface state changes of Ru and RuxSey particles and clarify the promoting role of Se during the ORR.

Surface-induced time-dependent instability of ZnO based thin-film transistors

We report on the surface-induced time-dependent instability of ZnO based thin-film transistors (ZnO-TFTs) with interdigitated source/drain (S/D) electrodes. As time elapsed, a considerable shift of threshold voltage ( V T) was observed (by ~ − 16 V) from our TFT. Contact angle of de-ionized water on ZnO surface also changed from 30° to 110°, revealing time-dependent surface state change. According to X-ray photoemission spectroscopy (XPS) measurements, the Zn 2p 3/2 core-level peak and the valence band maximum (VBM) of aged ZnO surface shifted to the higher binding energy by 0.3 eV, which implies a downward energy band bending of the ZnO back channel-surface. We conclude that without passivation layer any bottom gate ZnO-TFT meets the surface-induced electrical instabilities due to the time-dependent conductance of ZnO surface.

Optical Properties of the Carbon-Modified TiO2 Prepared by Microwave Carbonization Process

The carbon-modified TiO2 were synthesized through microwave carbonization of ethanol by using a domestic microwave oven. This process enabled to form the carbonaceous compounds on the surface of TiO2 and created several new mid-gap bands into the original bandgap within few minutes operation. The sample showed a remarkable visible-light absorption even at the wavelength of around 800 nm. The promotion of photocatalytic activity under visible and ultraviolet (UV) light irradiation were also confirmed by the I3- formation in KI aqueous solution. The I3- formation rate of carbon-modified TiO2 per unit mass under visible light is almost 25 times higher than that of pure TiO2. The mid-gap optical absorption mechanisms were investigated through analysis of absorption edges. It is revealed that surface state change against microwave-treatment time results in different mid-gap optical absorption processes.

Surface chemistry and catalytic activity of Ni/Al 2O 3 irradiated with high-energy electron beam

The radiation effects induced effects by electron beam (EB) treatment on the catalytic activity of Ni/γ-Al 2O 3 were studied for the carbon dioxide reforming of methane with different EB energy and absorbed radiation dose. Transmission electron microscope (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were used to determine the change in structure and surface states of Ni/γ-Al 2O 3 catalyst before and after the EB treatment. Higher energy EB treatment is useful for increasing the proportion of the active sites (such as Ni 0 and NiAl 2O 4-phase) on the surface. The increase of Ni/Al–ratio indicates that the Ni dispersion on the surface increased with the EB-treatment, resulting in an increase of the active sites, which leads to improving the catalytic activity. XPS measurement also showed a decrease of the surface carbon with EB dose. The maximum 20% increase in the conversion of CO 2/CH 4-mixture into CO/H 2 gas was observed for the catalyst treated with 2 MeV energy and 600 kGy dose of EB relative to untreated.

Gold Immersion Deposition on Electroless Nickel Substrates: Deposition Process and Influence Factor Analysis

An immersion gold-plating process on electroless nickel substrate was investigated. The effects of gold salt, trisodium citrate, bath temperature, and pH on the gold-immersion-deposition process are also discussed. The study was performed by measuring the mixed potential-time curves in situ and electrochemical impedance spectroscopy in combination with X-ray fluorescence spectrometry (XRF) and atomic force microscopy (AFM) surface analysis. Electrochemical measurements and XRF results show that both the deposition rate and the mixed potential changed during the gold-deposition process. These variations reflect the change of electrode surface state. AFM analysis shows that the morphology of nickel surface changed greatly at the initial stage of gold deposition. A model for understanding the gold-immersion-deposition process was proposed based on the experimental data. The effects of , trisodium citrate, pH, and temperature on gold-immersion deposition are significant, via affecting the gold film formation and the deposition rate. The optimal conditions of aqueous solution for the gold-immersion plating were determined. The experimental results support the explanation of the proposed model.

Effect of Humidity on Growth of Oxide Film on Surface of Copper Contacts

Contact surfaces are exposed to the atmosphere in general applications. Therefore, gases in the atmosphere such as oxygen and H 2 O are adsorbed on and react with the contact surface. Products formed on the surface such as copper oxide films degrade contact resistance characteristics. This surface contamination is an important problem for electrical contact applications. The author has studied the effect of humidification on contact resistance characteristics. In this paper, the effect of humidity on the growth of an oxide film on a copper surface was clarified. An increase in the humidity results in a decrease in the thickness, in contrast, a decrease in the humidity increases the thickness linearly. Changes in the oxide film thickness based on the level of humidity were measured by ellipsometry. Surface state changes influenced by humidification were analyzed topographically using a scanning tunneling microscope. The mechanism of the effect of humidity on the film thickness was discussed on the basis of the deduction of the copper oxide film by H 2 from the adsorbed H 2 O. Moreover, the changes in contact resistance levels for both static and sliding contacts due to humidity were measured, and a dependence on humidity was found.

Surface Treatment of Plastic Substrates using Atomic Hydrogen Generated on Heated Tungsten Wire at Low Temperatures

The surface properties of a plastic substrate were changed by a novel surface treatment called atomic hydrogen annealing (AHA). In this method, a plastic substrate was exposed to atomic hydrogen generated by cracking hydrogen molecules on heated tungsten wire. For the substrate, surface roughness was increased and halogen elements (F and Cl) were selectively etched by AHA. AHA was useful for pretreatment before film deposition on a plastic substrate because the changes in surface state relate to adhesion improvement. It is concluded that this method is a promising technique for preparing high-performance plastic substrates at low temperatures.

Surface resonance transition of roughened Cu(1 1 0)

The temperature dependence of the reflection anisotropy spectroscopy (RAS) of a Cu(1 1 0) surface has been studied over the temperature range 700–1000 K. Because of the roughening transition at 900 K, the bimodal feature at 4.2 eV for a clean surface shifted to 4.3 eV on annealing. A significant decrease in intensity of the same energy level was also observed with increasing annealing temperature. In the annealing temperature range 700–1000 K, anharmonic behavior is expected to be the predominant process of atomic disordering at the surface. Changes in the RAS of Cu(1 1 0) as a result of thermal processing can be understood in terms of the associated changes in surface states. The RAS signal for a surface resonance transition at 4.2 eV is associated with monoatomic [0 0 1] steps.

Preparation and Characterization of Graphite Powder Covered with CeO 2

In order to improve the wetting properties of graphite with Al melt and reduce the oxidation of the graphite, by which the segregation of components during the liquid-stir-casting process could be prevented. In this paper, a uniform thin nano-film of CeO 2, about 20 nm thick, was successfully prepared onto graphite powder surface by heterogeneous nu-cleation process. The results show that an obvious chemical reaction did exit between CeO 2 film and graphite with the formation of Ce-O-C bond, leading to a shift of the binding energy of C and Ce. The cover with CeO 2 film illustrates a distinct change of surface state of graphite with a decrease of angle of contact.